1. Field of the Invention
This invention relates to a digital communication receiving apparatus used suitably for a digital audio broadcast receiving apparatus.
2. Description of Related Art
DAB (digital audio broadcasting) has been known as a digital communication using phase modulation. The DAB is practically used according to EUREKA 147 standard in Europe, the signal processing on the transmission side is described herein under.
(1) A digital audio data having the maximum of 64 channels is compressed according to the MPEG audio layer II for every channel.
(2) Each channel data resulted from the above-mentioned (1) is subjected to error correction encode processing by convolution coding and interleaving of the time axis.
(3) The result of the above-mentioned (2) is multiplexed to one channel. When, auxiliary data such as PAD is added.
(4) The result of the above-mentioned (3) is subjected to interleave processing on the frequency axis and a symbol for synchronization is added simultaneously.
(5) The result of the above-mentioned (4) is subjected to OFDM (Orthogonal Frequency Division Multiplex) processing and subsequently subjected to D/A conversion.
(6) The main carrier signal is subjected to QPSK modulation (Quadrature Phase Shift Keying) depending on the result of the above-mentioned (5), and the QPSK signal is transmitted.
The DAB receiving apparatus may therefore have the structure as shown in FIG. 3, for example.
In detail, in FIG. 3, an antenna 11 receives a DAB broadcast wave signal, the received signal is supplied to a mixer circuit 15 through a signal line comprising, in the order of passing, a band pass filter 12, a high frequency amplifier 13, and bandpass filter 14, and a local oscillation circuit 16 supplies a local oscillation signal having the predetermined frequency which is variable depending on the received frequency to the mixer circuit 15, and the received signal is subjected to frequency conversion and converted to an intermediate frequency signal SIF having a predetermined frequency.
The intermediate frequency signal SIF is supplied to the mixer circuits 21I and 21Q through a band pass filter 17 for intermediate frequency filtration and amplifier 18 for intermediate frequency amplification. A local oscillation circuit 22 generates a local oscillation signal having a frequency equal to the intermediate frequency of the intermediate frequency signal SIF and having a phase which is different by 90 degrees from that of the intermediate frequency signal SIF, and the local oscillation signal is supplied to the mixer circuits 21I and 21Q. As described herein above, in the mixer circuit 21I and 21Q, the intermediate frequency signal SIF is subjected to frequency conversion and the intermediate frequency signal SIF is converted to an I signal SI and a Q signal SQ, and the signals SI and SQ are outputted.
The signals SI and SQ are supplied to gain control amplifiers 23I and 23Q, in which the signals SI and SQ are converted to signals Si and Sq having a predetermined level, and these signals Si and Sq are supplied to A/D converter circuits 24I and 24Q and converted to digital data DI and DQ. The data DI and DQ are supplied to an FFT (Fast Fourier Transform) circuit 31 through digital low-pass filters 25I and 25Q described herein after and subsequently through amplifiers 26I and 26Q and are subjected to OFDM demodulation, and the OFDM demodulated data is supplied to a Viterbi decoder circuit 32, in which deinterleaving and error correction are performed and a program (channel) is selected, and thus the digital audio data of the desired program is selected.
Subsequently, the selected data is supplied to an expansion circuit 33, in which MPEG data expansion is performed, the data expansion circuit 33 expands the digital audio data of the desired program to the data having the original data length and outputs it, the outputted digital audio data is supplied to a D/A converter circuit 34, in which the digital audio data is subjected to D/A conversion and is converted to an analog audio signal, and the signal is outputted to a terminal 35.
At this time, a programmable gain control amplifier which is capable of gain controlling with a digital control signal is used as the variable gain amplifiers 23I and 23Q. The signal DI and DQ from the amplifiers 26I and 26Q are supplied to level detection circuits 27I and 27Q, in which the signal level (the signal level obtained when the signal DI and DQ are D/A converted) of the signal DI and DQ is detected, the detected outputs are supplied to the gain control amplifiers 23I and 23Q as a gain control signal, and the signal SI and SQ supplied to the A/D converter circuits 24I and 24Q are controlled to a predetermined constant level.
Accordingly, the signal level of the signals SI and SQ to be supplied to the A/D converter circuits 24I and 24Q is maintained at a constant level which matches to the dynamic range of the A/D converter circuits 24I and 24Q even though the received signal level from the antenna 11 changes, and thus the signals SI and SQ are A/D converted correctly to the data DI and DQ.
The above-mentioned description is the outline of the DAB receiving apparatus.
In the conventional receiving apparatus described herein above, the digital low-pass filters 25I and 25Q are provided to compensate the band pass filter 17 for processing.
For example, as shown in FIG. 4A, if there is a disturbance signal SUD at a frequency (fD+xcex94f) near the broadcast wave signal SD (center frequency fD) desired to be received, for example as shown in FIG. 4B, the output signal from the mixer circuit 15 contains undesirably the signal component SIFUD which is resulted from the disturbance signal SUD through frequency conversion at the frequency (fIF+xcex94f) in the case of down heterodyne conversion in addition to the intermediate frequency signal SIF (center frequency of fIF) which is resulted from the desired wave signal SD through frequency conversion.
The inclusion of the disturbance component SIFUD in the output signal from the mixer circuit 15 as described herein above results in undesirably inclusion of the signal component SBBUD as shown in FIG. 4C generated from the disturbance component SIFUD through frequency conversion at the position of frequency xcex94f in addition to the I signal SI and Q signal SQ of the base band, and the disturbance component SBBUD affects adversely following data processing as a matter of course.
To remove the disturbance component SIFUD contained in the output signal from the mixer circuit 15, the band pass filter 17 having a passing characteristic as shown with a dashed line in FIG. 4B is provided on the step next to the mixer circuit 15 as described herein above.
However, the band pass filter is an analog circuit, therefore the center frequency and passing characteristic disperse. It is difficult to prescribe the temperature characteristic to a desired characteristic. Depending on the model, because the intermediate frequency fIF of the intermediate frequency signal SIF usually is in a range as high as from several ten MHz to several hundred MHz, if the disturbance signal SUD has a frequency near that of the intermediate frequency signal SIF, the dispersion becomes the more significant. Even if the band pass filter 17 were prescribed to a desired characteristic, such transmission receiving apparatus is disadvantageous in that parts cost is high and the apparatus size is large.
Because the band pass filter 17 can not remove the disturbance component SIFUD sufficiently, the digital low-pass filters 25I and 25Q are provided to remove the disturbance component SBBUD. In this case, because the low-pass filters 25I and 25Q comprise a digital circuit, required characteristic is obtained easily and stably. Further, the frequency of the I signal SI and Q signal SQ is low and, the disturbance component SBBUD is easily removed even if the frequency of the disturbance component SBBUD is near.
As described herein above, the processing performed by the band pass filter 17 is compensated by the digital low-pass filters 25I and 25Q, and the data DI and DQ from which the disturbance component SBBUD is sufficiently reduced is supplied to the FFT circuit 31.
However, to compensate the band pass filter 17 for the processing with the digital low-pass filters 25I and 25Q, A/D converter circuits having a wide dynamic range are required as the A/D converter circuits 24I and 24Q.
In detail, though C/N required to receive the signals SI and SQ is different depending on the communication system, it is assumed that 25 dB is required. As shown in FIG. 4A, it is assumed that the level of the disturbance signal SUD is higher than that of the desired signal by 30 dB. Because the level of the desired signal SD changes rapidly due to fading in an actual mobile communication, the dynamic range of the A/D converter circuits 24I and 24Q needs the margin more. It is assumed that the margin of 20 dB is required.
As shown in FIG. 5A, the A/D converter circuits 24I and 24Q need the dynamic range of at least 75 dB(=30 dB+25 dB+20 dB). To obtain the dynamic range of 75 dB, the A/D converter circuits 24I and 24Q need the number of bits of at least 13 bits.
However, if the disturbance component SIFUD is sufficiently reduced in the band pass filter 17, the dynamic range of 45 dB namely sum of C/N of 25 dB and fading margin of 26 dB is sufficient for the A/D converter circuits 24I and 24Q as shown in FIG. 4B, and the number of 8 bits is sufficient for the A/D converter circuits 24I and 24Q.
To sum up, to compensate the band pass filter 17 for the characteristic with the digital low-pass filters 25I and 25Q, the A/D converter circuits 24I and 24Q having a large number of quantization bits are required and, this method is not preferable because of high power consumption and high cost. To reduce the number of quantization bits of the A/D converter circuits 24I and 24Q, the bandpass filter of high performance is required and, this method is also not preferable because of high cost and large occupied space.
A system which is tested by adding a disturbance signal SUD having a level higher than a desired signal SD by 79 dB as GSM requires not only digital low-pass filters 25I and 25Q but also a filter to reduce the level of the disturbance signal SUD by 50 dB or more, for example, SAW filter. Such filter is expensive and large sized.
The present invention is accomplished to solve the problem described herein above.
In the present invention, the communication receiving apparatus for digital communication in which the frequency of received QPSK signal is converted to generate an intermediate frequency signal, the intermediate frequency signal is subjected to A/D conversion and I-component/Q-component separation to obtain I-component digital data and Q-component digital data, and the original digital data is obtained from the I-component digital data and the Q-component digital data has:
an A/D converter circuit for performing the A/D conversion;
a level control circuit for controlling the level of the signal supplied to the A/D converter circuit to a predetermined value; and
a detection circuit for detecting the level of a disturbance signal contained in the received signal;
wherein the reference level of the signal is controlled based on the output from the detection circuit correspondingly to the level of the disturbance signal so that the level of the signal supplied from the level control circuit to the A/D converter circuit ranges within the dynamic range of the A/D converter circuit.
The reference level of the signal supplied to the A/D converter circuit is therefore controlled to the level matched to the dynamic range of the A/D converter circuit, and then the signal is subjected to A/D conversion.